The prior art will be described below with respect to a hydraulic excavator for example.
According to the construction of a conventional hydraulic excavator, an upper rotating body is mounted rotatably on a lower traveling body, an excavating attachment comprising a boom, an arm, and a bucket is attached to the upper rotating body, and hydraulic oil discharged from pumps is fed to hydraulic actuators to effect booming, arming, bucketing, traveling, and rotating operations.
According to the construction of the conventional hydraulic excavator, however, the pumps are activated by an engine and pressure oil discharged from the pumps is fed to hydraulic actuators through control valves. Thus, a surplus flow in each pump is throttled and discarded into a tank through a control valve or a relief valve, thereby controlling the flow rate in the actuator concerned. With this construction, not only there arises a great loss of energy but also there arise problems related to environmental pollution such as the generation of noise and exhaust gas.
In view of this point there recently has been proposed what is called a hybrid type excavator wherein a generator is driven by an engine to rotate an electric motor and hydraulic pumps are rotated by the electric motor.
This hybrid type is advantageous in that the pump discharge rate (flow rate of oil fed to each actuator) can be controlled by controlling the number of revolutions of the electric motor and that therefore the loss of energy is basically small in comparison with the conventional pure hydraulic type.
However, since the proposed technique adopts the construction wherein plural hydraulic pumps are activated by one electric motor, the pumps are always equal in the number of revolutions despite of different quantities of oil to be discharged from them. Consequently, even a pump which is required to discharge only a small amount of oil comes to rotate at high speed following the rotation of the other pumps. Thus, the pump efficiency is low and the loss of energy increases because a surplus flow in each pump is discarded to the tank through a valve.
The following problems are also involved in the proposed technique.    1. During excavation there is performed, in many cases, a composite operation comprising an excavating operation using both arm and bucket and a boom raising or lowering operation which is conducted simultaneously with the excavating operation. At this time, both arm and bucket cylinders for performing a main excavating operation become high in pressure relatively, whereas a boom cylinder for raising and lowering the attachment does not become so high in pressure as both arm and bucket cylinders because of a great influence of the own weight of the attachment.
In this case, according to the prior art, since both boom cylinder and bucket cylinder are actuated by the same pump, it is required that the pressure of oil discharged from the pump, when increased up to the pressure of the bucket cylinder, be lowered with a control valve and then fed to the boom cylinder, thus causing a pressure (energy) loss.    2. Since there is adopted a construction wherein all of the boom, arm and bucket cylinders are controlled their operating speeds by a control valve opening control (open circuit control), a large gravity based on the weight of the attachment acting on those attachment components cannot be regenerated as power when brake a large gravity. Particularly, a large gravity acts on the boom which undergoes the action of the entire weight of the working attachment, but it is impossible to regenerate power during descent of the boom and thus also in this point there arises the waste of energy.
It may be proposed to adopt a construction wherein operating direction and speed are controlled by controlling the rotational direction and speed of an electric motor without using the control valve for each attachment cylinder. With this construction, however, the response characteristic at the time of switching extension and contraction of each cylinder from one to the other becomes deteriorated, so that it becomes impossible to effect works (mud removing work and earth and sand scattering work) which require a minute extension/contraction switching operation particularly for both arm and bucket cylinders.
In view of the above-mentioned problems, according to the present invention there is provided a construction machine of a hybrid type including electric motors to activate hydraulic pumps, which construction machine can eliminate a wasteful operation of the pumps and thereby attain the saving of energy.
According to the present invention there also is provided a construction machine of the above hybrid type, capable of ensuring a required response characteristic while suppressing the loss of energy.